Hydraulically-actuated, electronically-controlled fuel injectors and systems are known. Examples of such injectors and systems are shown in U.S. Pat. No. 5,460,329 to Sturman, U.S. Pat. No. 5,181,494 to Ausman et al., and U.S. Pat. No. 5,682,858 to Chen et al.
In the design alternative depicted in FIG. 6 of Sturman, the back of the needle valve is fluidly coupled directed to the high pressure actuating fluid source. It is significant to note that this embodiment does not utilize a spring to close the needle valve. The intention of the embodiment is to eliminate the needle valve spring and to use only actuating fluid rail pressure to close the needle valve. In this embodiment, there is no means for amplifying the actuating fluid pressure acting at the back of the needle valve. The needle valve front and the needle valve back have equally sized pressurized areas. A deficiency of this design is that the needle valve may have uncontrolled opening (since there is no valve spring to maintain the needle valve in the closed condition) when combustion cylinder pressure acting on the needle valve is relatively high and when the actuating fluid common rail pressure is relatively low, for example, during engine cranking or low speed engine operation.
Chen et al. incorporates a needle valve control chamber. The fluid pressure in the control chamber is directly controlled by the injector solenoid valve. The solenoid valve exposes the chamber to either the pressure in the actuating fluid high pressure rail or to ambient pressure as a function of solenoid valve position. When the control chamber is vented to ambient, the needle valve opens by fuel pressure acting counter to the relatively small needle spring. Such an arrangement indicates that the needle opening pressure in all cases is disadvantageously at the relatively low fuel injection pressure necessary to overcome the bias of the relatively small needle valve spring. The disadvantage of this design carries across the entire engine speed and load range. When the needle valve control chamber is exposed to the actuating fluid rail pressure, the needle valve closes by the total force of the actuating fluid acting on the needle valve and the force of the needle valve spring. This needle valve closing force can be very great at high actuating fluid rail pressure. The rail pressure force is amplified by the piston in the needle valve control chamber acting on the back of the needle valve.
Typically, in the conventional prior art HEUI type injector shown in Ausman et al., needle valve operation is controlled by a fixed mechanical return spring opposed by a force generated by fuel pressure acting on the needle valve. The preload on the conventional spring is predefined. Accordingly, the needle opens and closes at fixed fuel pressures under all engine operating conditions. Selecting the return spring load involves some tradeoffs between high speed high load operability and low speed operability. If the prior art return spring load is selected based on the rated engine condition performance requirements, then, the return spring load could be too great for lower speed conditions, especially idle conditions. High valve opening pressure produces significantly greater engine operation noise, a particularly undesirable effect. At engine idle condition, with a heavy spring, the engine operation noise becomes even more pronounced. Reducing diesel engine idle noise is a critical challenge to make the diesel engine acceptable for use in family transportation vehicles, such as pickup trucks and SUV's. Reducing idle noise is a key for the diesel engine manufacturer to be able to compete in what is now a largely gasoline engine market. The low valve opening pressures of the present invention offer a significant competitive advantage.
There is a need in the industry to provide a hydraulically-actuated, electronically-controlled fuel injector and system with variable needle valve valve opening pressure. The mechanization necessary to provide such variable valve opening pressure should be designed in the most simplistic way possible in order to minimize the difficulty in constructing the injector, minimize the complexity of the injector, and in order to minimize the cost of the injector.